Piston alloy



Patented Sept. 7, 1937 UNITED STATES PISTON ALLOY Russell H. McCarroll,Dearborn, Mich., assignor to Ford Motor Company, Dearborn, Mich, a

corporation of Delaware No Drawing. Application January 30, 1935,

Serial No. 4,126

1 Claim.

The object of my invention is to provide a steel alloy especiallyadapted for constructing internal-combustion engine pistons, whichpistons will have most of the advantages of aluminum pistons withoutseveral of the disadvantages inherent in such aluminum pistons. Aluminumpistons are advantageous in that the metal aluminum is anexceptionallyogood heat conductor so that the transfer of heat from onepart of the piston to another part is rapid enough to allow the pistonto remain cool even though comparatively high compression pressures areused. Furthermore, aluminum being very light in weight minimizes thereciprocating strains on the wrist pin and connecting rod bearings, andalso requires aminimum of counter-weighting in the crank-shaft toproducea running balance. These advantages are sumciently important tomake an aluminum piston preferable to the ordinary cast iron piston forhigh speed engine use. However, they are obtained only with certaininherent disadvantages. These disadvantages are that aluminum has a highco-efiicient of expansion so that compensating slots, invar struts, orsome other means must be provided to prevent piston slap during thewarming up period of the motor. Furthermore, aluminum has acomparativelyhigh co-efficient of friction with cast iron so thatexcessive piston wear results, and further the cylinder surface attainsa somewhat hotter temperature than when cast iron pistons are used, dueto the increased friction.

Perhaps the greatest disadvantage inherent' in aluminum pistons is thatthe metal is comparatively soft so that the piston ring grooves poundout or increase in width appreciably after only a few hundred hours ofuse. Furthermore, wear on the piston skirt causes the piston to wobbleas it reciprocates, which motion wears off the sharp edges of the ringgrooves. These two conditions cause the engine to pump considerable oil.One of the reasons for oil passing by the pistons in internal-combustionengines is that excessive clearance between the ring grooves and therings, allows the oil upon the down stroke of the piston to be forcedinto the space inba'ck of the piston rings so that upon the succeedingdown stroke of the piston this oil is deposited on the cylinder wallsabove the ring: To prevent oil pumping, itis essential that there be aminimum clearance between the ring grooves and the piston rings. Theapplicants piston alloy has substantially the same co-efficient ofexpansion as have cast iron piston rings and it is sufnot becommercially machined.

ficiently hard and wear resistant that the ring grooves are preventedfrom appreciably increasing in width. For this reason the rings, evenafter hundreds of hours of use, fit very closely in the ring grooves.

The applicants metal is believed unique among steel alloys in that it isan exceptionally good heat conductor to thereby allow a minimumthickness of the piston head. The improved heat conductivity is obtainedfrom a copper matrix which is provided in the alloy.

Still further, the applicants alloy is an exceptionally free flowingmetal to thereby permit the commercial casting of relatively thin ribsand piston walls.

Still further, the sulphur content is particularly high in theapplicant's alloy, it being three to four times as high as the maximumusually permissible in cast-iron. Experience has shown with thiscombination of elements a high sul- 20 phur content provides for freermachining which is essential when thin wall castings are to be machined.

It is believed that all of these characteristics are essential inasmuchas without an increased 25 heat conductivity, a piston having a lightweight piston head, ribs and skirt, could not be used 'in highcompression motors, and without free flowing of the metal, such lightweight construction could not be cast, and without freer machining 30characteristics, such thin walled pistons could With these and otherobjects in view, my invention consists in thecomposition and combinationof elements in my improved alloy, as 35 described in the specificationand claimed in my claim.

My improved alloy consists of the following:

Carbon 1.40-1.70 per cent Manganese .90-1.10 per cent 40 Sulphur .12-.16 per cent Silicon .90-1.10 per cent Copper 1.50-2.00 per centPhosphorus .12 (max) Iron balance 5 a Brinell of 07-286 is readilyattained, the ma- 1 terial having a tensile strength of about 100,000

pounds per square inch.

With the above mentioned heat treatment, the carbon is in the form oftemper carbon and not as graphite flakes. The abovementioned heattreatment differs from the conventional normalizing treatment in thatheating the castings to 1650 F. breaks up the grains and startssecondary graphitization, while the 1400 draw completes thisgraphitization while at the same time spheroidizing the pearlite. Inthis way secondary graphite is formed which result is not obtained bynormalizing. This accounts, to a great extent, for the increasedstrength of the alloy and the increased wear resistance of the metal.Furthermore, a characteristic of my improved alloy is that the copper isheld in solution by the silicon. I

An important advantage obtained with my improved piston alloy is thatthe copper content forms a matrix or network throughout the castingwhich improves the heat conductivity to such extent that thecross-sectional area or thickness of the piston head may be reduced tosubstantially that required to carry the structural load upon the head,Inasmuch as this material has a tensile strength of approximately100,000 pounds per square inch a comparatively thin piston head, muchthinner than required with cast iron pistons, is sufficient to carry thestresses imposed on same, and due to the improved heat conductivity;pre-ignition does not occur. Excellent results with no pre-ignition havebeen obtained using a three inch diameter piston having a head-thicknessof only .090 and operating at 6.5 atmospheres of compression; whereas,when an ordinary cast iron piston is used, the piston head must beseveral times this thickness to give the necessary structural strength.If an ordinary steel piston is used, over 50% increase in head thicknessmust be provided to prevent pre-ignition, due to the lower heatconductivity of steel.

A further advantage obtained with my improved alloy is that thecoefiicient of friction with a cast iron cylinder is considerably lessthan the co-efilcient of friction between cast iron and cast iron andconsiderably less than the co-eflicient of aluminum on cast iron.Consequently, less wear results on the piston walls and less heat is inginternal-combustionv engine pistons, comprising,

Carbon 1.40-1.70 per cent Manganese .90-1.10 per cent Sulphur .12- .16per cent Silicon .901.10 per cent Copper 1.50-2.00 per cent Phosphorus.12 (max Iron balance RUSSELL H. MCCARROLL.

